Polymer emulsions or polymer latices prepared by conventional emulsion homo- or co-polymerization of vinyl acetate using polyvinyl alcohol as protective colloid are widely used for adhesives. There are several application fields for such emulsions such as in wood glues, paper and packaging adhesives (e.g. case sealing, book binding, bag making, card board lamination), parquet adhesives and others, most importantly woodworking adhesive formulations.
All these adhesives and especially woodworking adhesives are optimized with respect to several properties such as water resistance of the bond, heat resistance of the bond and creep of the bond. Further, before application several dispersion properties must be met. These are compatibility with formulation aids such as film forming agents, fillers, metal salts; theological properties; storage stability; the speed of set and so on.
Vinyl acetate based polymer latices for woodworking adhesives having good water resistance typically use some functional post-crosslinking comonomer in polymerization and the formulation with a catalyst after polymerization. Amongst the post-crosslinkers N-alkylol derivatives of an amide of an xcex1,xcex2-ethylenically unsaturated carboxylic acid, such as N-methylol acrylamide (NMA) which is preferred for latex wood glues are known. For example, U.S. Pat. No. 5,439,960 and U.S. Pat. No. 5,391,608 describe the use of 0.1 to 6% of NMA based on total monomer content. Likewise European Patent Application EP-A-0,561,221 describes the use of 2 to 10% NMA.
As the catalyst for formulation of the above woodworking adhesives having good water resistance water-soluble metal salts of monobasic acids such as aluminum(III), iron(III) and/or zirconium(IV) salts, especially aluminum nitrate, iron chloride or zirconium oxy-chloride are used (see for example DE-26,20,738, EP-A-0,433,957, EP-A-0,501,174, EP-A-0,686,683 and U.S. Pat. No. 5,434,216).
Another approach to improve water resistance of wood glues is for example disclosed in EP-A-0,433,957, EP-A-0,623,661 and JP-06,322,337 which documents relate to use of more hydrophobic comonomers such as vinyl pivalate, diisopropyl maleate, diisopropyl fumarate, vinyl chloride or versatic acid vinyl esters for this purpose.
U.S. Pat. No. 5,296,532 discloses a vinyl ester graft copolymer comprising 2 to 12% by weight of one or more polyvinyl alcohols and 88 to 98% by weight of a graft monomer mixture. The graft monomer mixture comprises 5 to 50% by weight of a saturated aliphatic highly branched mono-carboxylic acid having 9 to 10 carbon atoms, 50 to 95% by weight of a vinylic ester having 1 to 18 carbon atoms in the ester residue, and 0.01 to 4% by weight of a copolymerizable polyethylenically unsaturated monomer such as divinyl adipate, triallyl cyanurate or diallyl maleate. The graft copolymer may further comprise up to 26% by weight of other monoethylenically unsaturated monomers. The document examplifies a vinyl acetate based woodworking adhesive or wood glue comprising 0.03% triallyl cyanurate.
U.S. Pat. No. 5,391,608 and 5,439,960 disclose a two-stage emulsion polymerization procedure to obtain a graft copolymer according to which two-stage process in the first stage vinyl acetate is emulsion polymerized with 0.1 to 6% by weight of a post-crosslinking monomer in the presence of polyvinyl alcohol and in the second stage methyl methacrylate is emulsion polymerized with 0.1 to 6% by weight N-methylol acrylamide and 0.01 to 0.5% by weight triallyl cyanurate or diallyl maleate to obtain the outer shell of the graft polymer.
Adhesive performance of for example a wood glue is typically controlled by the amount of polyvinyl alcohol, i.e. the ratio of polyvinyl alcohol to monomer, the polyvinyl alcohol molecular weight and its degree of hydrolysis as for example disclosed in U.S. Pat. No. 5,434,216. In addition, functionalized polyvinyl alcohols, e.g. acetoacetylated polyvinyl alcohols (JP-A-06,279,644) or itaconic acid-modified polyvinyl alcohols (JP-A-07,11,217) may be used.
The choice of the polyvinyl alcohol (PVOH) in turn determines other properties of the vinyl acetate latex and thus the adhesive obtained, especially its rheological properties such as viscosity and shear thinning, or the speed of set. Rheology thus will mainly depend on the molecular weight distribution and degree of hydrolysis of the polyvinyl alcohol used. For example, EP-A-0,686,683 discloses altering the theological properties by altering the formulation with some thickening agent, thereby changing the continuous phase viscosity.
None of the above techniques allows for a simple straight forward independent control of these properties. It is therefore the object of the present invention to find a possibility for independent control of the shear thinning and speed of set, which in turn allows to precisely and intentionally adapt the emulsion or latex for certain applications thereby improving performance of for example a woodworking adhesive or wood glue.
Nevertheless, it would be especially useful and desirable to provide a certain shear thinning behavior and to improve the speed of set. At the same time, however, the viscosity level should remain more or less unchanged like the other useful adhesive performance parameters, created e.g. by the level of NMA comonomer or other comonomer and/or by the molecular weight and hydrolysis distribution of the PVOH protective colloid.
The above object is solved and the drawbacks of the prior art are overcome according to a first aspect of the invention by a vinyl acetate based polymer latex composition obtained by emulsion polymerization, the polymer comprising (i) PVOH, and (ii) a monomer mixture, said monomer mixture comprising (a) vinyl acetate, and (b) a pre-crosslinking monomer selected from one or more polyethylenically unsaturated compounds wherein said pre-crosslinking monomer is present in amount of less than 0.01% by weight based on the total monomer content and which pre-crosslinking monomer is initially charged to the polymerization reaction mixture.
The vinyl acetate based polymer latex composition may further comprise a post-crosslinking monomer as well as other copolymerizable monomer selected from vinylic esters, acrylic monomer, carboxylic acid and ethylene.
In a second aspect, the present invention relates to an adhesive comprising a vinyl acetate based polymer latex composition obtained by emulsion polymerization, the polymer comprising (i) PVOH, and (ii) a monomer mixture, said monomer mixture comprising (a) vinyl acetate, and (b) a pre-crosslinking monomer selected from one or more polyethylenically unsaturated compounds wherein said pre-crosslinking monomer is present in amount of less than 0.01% by weight based on the total monomer content and which pre-crosslinking monomer is initially charged to the polymerization reaction mixture, (c) further optionally comprising a post-crosslinking monomer, up to 40% by weight of other vinylic esters, up to 15% by weight of acrylic monomer, up to 3% by weight of unsaturated carboxylic acid, and up to 20% by weight of ethylene, all percentages being based on total monomer content, optionally in combination with appropriate additives. Preferably the adhesive is a woodworking adhesive.
According to a third aspect, the present invention relates to use of the above vinyl acetate based polymer latex composition optionally in combination with appropriate additives in an adhesive, preferably a woodworking adhesive.